1. Field of the Invention
This invention relates, in general, to a switching circuit for testing custom made Large Scale Integrated (LSI) circuits and more particularly to a bidirectional switching circuit for measuring a large number of AC data paths of macrocell arrays.
2. Background Art
To satisfy the demand for large scale digital integrated circuits, the semiconductor industry has developed three basic approaches. These include standard, off-the-shelf circuits; custom circuits; and gate arrays. The standard, off-the-shelf circuit provides the lowest cost option due to the quantities manufactured, but are limited in providing the flexibility for the circuit desired. The custom circuit is cost limiting unless the number of circuits desired is large. The gate array involves a standard array of a large number of gate circuits diffused into a chip. The metallization pattern converting these gate circuits into functional custom circuits is processed according to the customer's requirements.
A macrocell array is an extension of the gate array concept. Each cell in an array contains a number of unconnected transistors and resistors. A metallization interconnecting pattern transforms the interconnected transistors and resistors within each cell into Small Scale Integrated/Medium Scale Integrated logic functions, called macros. The macros take the form of standard logic elements such as dual type "D" flip flops, dual full adders, quad latches, and many other predefined functions. The macros are also interconnected by the metallization to form the desired LSI design. The high density packing of a macrocell array chip offers up to a fifty to one reduction in system component count, with a power dissipation improvement of as much as five to one.
These macrocell arrays are manufactured by modern technological processes to very precise standards. The propogation delay along AC data paths is measured in terms of nanoseconds. In order to insure user specifications are met and to confirm that the macros form the desired logic function, a testing sequence, or quality evaluation program, is normally conducted on each macrocell array produced. One known method of testing macrocell arrays involves the use of a Sentry VIII tester, produced by Fairchild Test Systems, probably the most advanced LSI general purpose tester that provides a large number of input/output (I/O) channels. The Sentry VIII tester is a computerized part tester that probes AC tests with 120 I/O channels and with a resolution of 160 picoseconds. There are seven pulse generators for producing pulses of different formats. The part being tested is placed on a load board having leads and terminals for coupling the part to the Sentry VIII. The term AC when used in AC tests refers to a signal with input to output transitions. Conversely, DC tests involve steady state currents and voltages having a high low state.
However, the overall accuracy of the Sentry VIII tester when measuring AC data paths of an LSI device is plus or minus three nanoseconds. Since some AC data paths on the device may have actual delays of only two nanoseconds and internal delays of less than one nanosecond, it would be beneficial to improve the accuracy of the testing method. Some of the reasons that the Sentry VIII is limited to an accuracy of three nanoseconds are as follows:
1. The general purpose testing of arrays requiring different inputs and outputs on various options sacrifices a transmission line environment by not matching input and output resistances.
2. The large number of I/O pins makes the load board difficult to design. "Open" lines must be used on inputs instead of terminated lines. Different lengths of line are used. Relays and other external components are required on the load board which add capacitance.
3. The rise and fall times of the drivers are slow (3-5 nanoseconds). Rise times of one nanosecond are used in laboratory measurements.
4. The output comparators used for measuring device outputs to a known reference voltage (threshold) has a response time dependent on the rise and fall times of the device being tested.
5. The pulse generators used to generate input signals and strobe pulses have skews between them due to differences in channel drivers.
6. The pulse generators in the Sentry VIII tester can be incremented in 0.16 nanosecond steps which means that repeated AC readings on the same device is .+-.0.16 nanosecond.
The accuracy of LSI testing has previously been improved to one nanosecond using a "Golden Unit/Silver Unit" approach. First, the device under test (the golden unit) is AC tested using a digital sampling scope while the circuit is in the load board on the Sentry tester. The Sentry tester measures the golden unit. The difference of the two readings for each measurement is stored in a deskew table. After this procedure is completed on all AC tests, several other units or macrocell arrays of the same option (called silver units) are tested with the Sentry tester and a deskew table is generated for each of the silver units. The silver units are used to deskew the machine prior to testing on a regular basis. By deskewing the machine, it is meant that the values read for each unit tested are adjusted according to the difference between the values read for the silver unit when first measured and just prior to testing each unit or group of units. For each option developed on the macrocell array, a golden unit must be measured manually (normal measurement procedure requires 3-4 hours per option) and about nine silver units must be tested and stored for deskewing the machine for that option only. This means that 10 units must be tested and stored for every option developed. Silver units are used for production testing since if the silver units get damaged, replacement silver units can be made from the gold unit without manually taking measurements.
For example, if 200 options of a macrocell array have been developed, 200 golden units must be measured and deskew tables generated for 1800 silver units. Obviously, this is a lot of units to be tested manually and stored for production testing. These problems are compounded as macrocell array options may number in the thousands. Since the amount of manual testing required is large, each option developed is limited to only a small number of AC paths for production testing.
Thus, what is needed is a bidirectional switching circuit that improves the accuracy and reduces the time required for testing different options of a macrocell array.